Topic: 73' Honda cb500 four k2- Clyinders 3&4 not firing

[Tunnelvision111]

Just picked up a cb500 for a cafe racer project and decided to start with the engine.....not doing so good.

Here's the facts:

- new plugs
- new condensers
- points tuned
- coils tested and are strong (10+mm spark gap)
- all plugs getting spark
- all valves are opening (although I'm not sure if correctly-tappets)
- using a "quick start ether spray" in the carbs to get her fired up.
- suction on intake side a little weak on 3&4 compared to 1&2.

It will fire up but only for 3-5 seconds. After it dies I can hear a vacuum sucking sound like pressure is trying to equalise itself (from where I dont know)

When I pull all four plugs to have a look, 1 & 2 are blackened, but 3&4 are still brand new.

I have no clue what's going on because 1&4 and 2&3 run on the same coils, condensers, and points.
And ideas of where I should start looking?

Cheers

[LesterPiglet]

Do a compression test. Sucking noises are not good.

[bryanj]

One petrol pipe feeds 1 & 2 the other 3 & 4 so possibly a bunged up petcock outlet or pipe or even stuck floats.

Remove the floatbowl drain screws to see if gas is getting there---if it is try cleaning the pilot jets

[Tunnelvision111]

Also I should mention she backfires and often blows back. Some pretty sweet fire balls gotta be a bad sign.